A thermal recording system capable of providing a record image upon input of a signal is widely used in facsimiles, computer terminal printers, printers for measuring instruments, etc., since the device used in the system is relatively easy to handle, low in noise and available at an inexpensive cost.
As the recording medium to be used in such thermal recording system, there is commonly known a thermal recording sheet of a so-called color formation type which is provided with a recording layer capable of undergoing physical and chemical changes to cause color formation by application of heat. However, there are disadvantages for this thermal recording sheet that it is liable to induce undesirable color development either during the fabrication process thereof or during storage thereof; and the image formed thereon is poor in storage stability and is apt to fade upon contact with organic solvents or chemicals. In order to eliminate these problems, there has been proposed a thermal recording system in which a recording medium utilizing a self-colored coloring material is used instead of the color-developing thermosensitive recording sheet. For instance, Japanese Patent Laid-open 51(1976)-15446 discloses a recording system in which a substrate, such as paper, polymer film, etc., coated with a coloring material which is solid or semi-solid at room temperature is superposed on an image-receiving sheet in such a manner that the coloring material on the substrate comes into contact with the image-receiving sheet, and the coloring material on the substrate is heated by a thermal recording head and selectively transferred to the image-receiving sheet to thereby obtain a recorded image.
According to this recording system, the coloring material on the substrate is melted, evaporated and sublimated by application of heat, followed by transferring to the image-receiving sheet to form an image thereon through sticking, adsorption and dye-fixing. One of the admitted features of this recording system is that plain paper may be used as the image-receiving sheet. And, a recording system using a sublimable dye as the coloring material provides an image excelling in resolution. Hence attempts have been made to apply this recording system to full color recording. However, when plain paper is used as the image-receiving sheet, there are disadvantages that the dye-fixing, in particular, is hardly attained and because of this, not only the image recorded becomes poor in color density but also distinguished fading occurs for the recorded image with the passage of time. In order to eliminate these disadvantages, there has been proposed the use of an image-receiving sheet having an image-receiving layer mainly comprising a thermoplastic resin as disclosed in Japanese Patent Laid-open 57(1982)-107885 or U.S. Pat. No. 3,601,484.
Now, the provision of such image-receiving layer mainly comprising a thermoplastic resin provides improvements on recording sensitivity and storage stability to some extent, but there still remain some problems to be solved. For instance, the process of forming the image-receiving layer by dissolving a thermoplastic resin in a solvent is accompanied by not only problems in working efficiency but also a fear of fire disaster. In addition, in the case where a water-soluble or water-dispersible resin is used for the formation of the image-receiving layer, the image recorded on the resultant image-receiving layer is liable to fade when stored under a highly humid environment because of the constituent resin of the image-receiving layer which is likely to absorb moisture from the air. There are other problems in this case that cross-linking agent or/and reactive modifier contained together with the water-soluble or water-dispersible resin in a coating composition for the formation of the image-receiving layer is often reacted with water or reacted with the water-soluble or water-dispersible resin prior to applying the coating composition onto a base sheet or subjecting the coated material to air-drying. Because of this, it is difficult to obtain a stable coating composition which is free of changes in properties and which is capable of providing a desirable image-receiving layer. In order to eliminate these problems, there is a proposal to modify the water-soluble or water-dispersible resin prior to use. However, this proposal is problematic since there is a limit for the kind of an usable resin and, in addition to this, the number of the steps of forming the image-receiving layer is unavoidably increased.
Other than the above, in order to improve the physical properties of the foregoing thermoplastic resin, Japanese Patent Laid-open 61(1986)-277493 or 62(1987)-238791 proposes to make it cross-linked. Further, a number of proposals have been made aiming at improvement in the printability or prevention of fusing with respect to the image-receiving layer. For example, Japanese Patent Laid-open 60(1985)-212374 proposes incorporation of a releasing agent into the image-receiving layer in order to prevent the surface of the coloring material-containing layer of the transfer sheet from being fused with the surface of the image-receiving layer of the recording sheet, resulting in making it impossible to record. Other than this, Japanese Patent Laid-open 62(1987)-222895 proposes use of a resin modified with silicone oil or the like in the preparation of the image-receiving layer.